You have to budget your time. I like half hour segments, but you may find something else works better. For me I tell myself that for this particular half hour, all I’m going to do is study this concept I want to study. I have a plan for the day where each free half hour segment that I have is set up ahead of time. Of course making the plan is one thing; actually following it is another. That would be another Quora answer, and my solutions for that may not work for you.

The point is that you must devote yourself to the thing you want to learn for at least some span of time. The more time you devote the better, although you may find some diminishing returns if you get too obsessive. You may have heard how many of the best musicians played until their fingers bled; the best programmers spend all night on code surviving on cheesy poofs and energy drinks. These stories may be true, but sometimes passion doesn’t come as naturally. Sometimes you have to cultivate it. So for half an hour, or whatever you can manage, attack the concept with all the will you can muster. After the time is up, you have permission to stop.

Devoting yourself to learning the concept means, no internet beyond what you need, no texting, no talking to friends, no video or TV watching, and no distracting music. You may have a playlist of songs that help you concentrate. I like instrumental music with a dance or techno rhythm sometimes for that, but most of the time I find I’m so worried about whether or not I can work with a particular song, that I just learn to deal with silence. This can be difficult, even dangerous for some people. Your job may depend on you being available. But if you want to focus on something, you are going to have to let something go for a little while. If you can’t find some time to devote to this thing you want to learn, I’m afraid you aren’t going to be able to learn it.

Now, after you’ve picked a concept, and devoted yourself to it, the next thing you need to do is step away from it. Ask any writer who has managed to overcome writer’s block. Sometimes obsessing over something, especially something complicated and difficult to understand, just does not work. Sometimes you have to give the logical part of your mind a rest. Do something else for a while. You may need to schedule this you-time just as stringently as your concept-time, but it helps. Although it’s true that learning the concept will be it’s own reward, the human brain wants instant gratification. After the third time you read about some one arguing over whether there’s six dimensions or eleven, you are very likely going to want to say “screw string theory” and eat a hot dog.
(There’s nothing difficult to understand about a hot dog. You can ponder it’s origins and chemical make up if you want, but it will only last a second before you tell yourself “Dude, it’s a freaking hot dog!” Much like religion, hot dogs require faith, but with faith comes solace, and you can make yourself one with everything.)
Wait until your devotion time is up, but then go for it. Take it easy for a while. After you’ve worked on it for the duration you set for yourself it will feel like a reward, and you’ll be more likely to want to study again. Don’t forget you’re human. Don’t burn yourself out.

Finally, for all the times when you aren’t intensely concentrating on some concept, you should remember to be open to serendipity. You may suddenly understand why there has to be six (or eleven) dimensions while you’re sitting in traffic on your way to work. It all has to do with rollerskates, you realize. Suddenly your boss calls. You pick up your phone. You could answer it right away, or you could text “rollerskates” to yourself while traffic is stopped. Text those rollerskates. As long as it’s not going to get you fired to let the phone ring a little longer, let your brain work in it’s weird and wonderful way. Remember that you care about string theory. It’s important to you. Later on, when you’re diving into it again and you’re lost you’ll remember you texted yourself something and then you’ll go “Oh yeah, it’s like rollerskates!” and you’ll be back on track.
So in review: